Geothermal power system

ABSTRACT

Disclosed are a system for extracting heat from hot unrefined water for the purpose of using this heat to do useful work, and specifically for extracting such heat while minimizing the undesirable effects caused by formation of scale and other solid build-up of scale-forming impurities in the hot unrefined water. Before contact with a heat exchange surface the hot unrefined water has added thereto an agent capable of increasing the formation of non-scale-forming species of said scale-forming impurities. These agents may be added immediately before passing the hot unrefined water through heat exchange equipment and/or directly into a geothermal well.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division, of application Ser. No. 513,777, filed Oct. 10,1974, now U.S. Pat. No. 3,951,794 granted Apr. 20, 1976, which is acontinuation-in-part of U.S. patent application Ser. No. 424,470, filedDec. 13, 1973, now U.S. Pat. No. 3,935,102 granted June 27, 1976.

BACKGROUND OF THE INVENTION

The present invention relates to a system for extracting heat from hotunrefined water for the purpose of utilizing this heat to do usefulwork, and more especially, to a system and method for extracting heatfrom water from a geothermal source wherein the disadvantageous effectsupon the heat extraction system as a result of scaling and other solidbuild-up of impurities contained in the unrefined hot water aresubstantially avoided.

In the course of the presently ensuing search for additional andimproved sources of energy to meet rapidly growing demand, investigatorsare studying the feasibility of utilizing naturally available energysources such as, for example, naturally heated water from geothermalsources. This source of energy provides the additional advantage that itis nearly pollution free, since after absorption of its heat, the watercan be returned to the ground leaving no polluting by-products.

One of the most significant problems, however, associated withextraction of useful heat from geothermically heated water resides inthe fact that the water often contains large amounts of impurities, bothin solution and in suspension. This water is referred to herein asunrefined water and is often saturated or supersaturated with impuritiessuch as silica, calcium sulfate, silicates and other compounds. Otherimpurities such as silica, silicates and iron may be present in the formof collodial dispersions in the water. These impurities give rise tosevere problems of scaling, corrosion, etc., on the surfaces of theapparatus utilized to transport the water from the underground sourceand, even more particularly, the heat exchange apparatus utilized forextracting heat from the water. Purification of the unrefined waterprior to extracting its heat is unreasonably expensive and decreases theefficiency of geothermal power sources.

In co-pending U.S. patent application Ser. No. 424,470, of which thisapplication is a continuation-in-part, there is described a novel systemand method for extracting heat from hot unrefined water with theavoidance of detrimental effects caused by impurities contained in thewater. According to the disclosure of that application, the hotunrefined water is prevented from coming into direct contact with thesurfaces of the heat exchanger used to boil a working fluid. Filters andlike equipment are not needed. A heat transfer medium in the form of ahousing containing porous material such as a bed of gravel or othergranular material is used to transfer heat from the unrefined water toclean water which is then passed through the heat exchanger. Such a heattransfer medium will be referred to herein as an accumulator-type heatinterchanger. The porous material is inexpensive and expendable and caneven be easily cleaned and reused if desired.

In the system a volume of the hot unrefined water is passed through ahousing containing porous material which picks up the heat of the water.A volume of clean water is then passed through the housing to pick upthe heat from the porous material. The now heated clean water can thenbe passed through a heat exchanger without significant danger to thesurfaces of the exchanger. The clean water can be recycled through thesystem many times, each time passing through the housing immediatelyafter a volume of the unrefined water.

In another important aspect of the system disclosed in the co-pendingapplication, the source of the clean water may be the unrefined waterwhich has been passed through the porous material. After being removedfrom the housing, the cooled unrefined water is delivered to a detentionreceptacle. Here it attains stabilization as many of the impuritiessettle to the bottom of the receptacle. The liquid which is left on thetop of the receptacle is substantially free of impurities to the extentthat what impurities are left in the liquid are not sufficient to undulydamage the surfaces of the heat exchanger. It is this substantiallyimpurity-free liquid which is used as the clean water, yet no filtering,etc., is necessary.

A preferred embodiment of the system disclosed in the co-pendingapplication provides for continuous operation of the system by the useof two housing containing porous material. The source of hot unrefinedwater is connected to the entrance end of one of the housings and hotunrefined water is passed through this housing until the leading edge ofthis volume of water is at the exit end of the housing. At this time,the source of hot unrefined water is disconnected from the first housingand connected to the entrance end of the second housing; simultaneouslythe source of cool, clean water is connected to the entrance end of thefirst housing (having been previously connected to the second). At thesame time that the source of clean water is connected to the firsthousing, i.e. when the leading edge of the preceding volume of unrefinedwater has reached the exit end of the first housing, the exit end of thefirst housing is connected to the detention receptacle so that theunrefined water may be deposited therein. Meanwhile, the leading edge ofa volume of clean water, which water has been heated, has reached theexit end of the second housing which is then connected to the heatexchanger. Thus, the entrance ends of the housings are alternatelyconnected to the sources of unrefined and clean water and each time theconnections at the entrance ends are switched, the connections at theexit ends are also switched to alternately direct unrefined and cleanwater from the housings to the detention receptacle and the heatexchanger respectively. To allow for proper timing in this simultaneousswitching of the connections of the entrance and exit ends of the beds,a temperature front, on one side of which the porous material and waterare at their highest temperature and on the other side of which theporous material and water are at their lowest temperature, must movealong the porous material with half the velocity of the water volumesuch that it is at the center of the housing when the leading edge of avolume of water has reached the exit end. One way of achieving this isto choose the porous material such that its heat capacity per unitvolume when dry is substantially the same as that of the unrefined wateror clean water contained in the voids of a unit volume of the gravel orother porous material.

Further details of the system and method disclosed in the co-pendingapplication will be understood from the disclosure of that applicationwhich is hereby specifically incorporated by reference and is reliedupon.

It was discovered in accordance with the invention disclosed in theco-pending application that the small concentration of particlesremaining in suspension in the substantially impurity-free liquidwithdrawn from the detention receptacle as a source of clean water forthe heat exchanger gives rise to a beneficial effect, namely, theirpresence inhibits the formation of scale upon the surfaces of the heatexchange apparatus. This phenomenon is believed to be the result ofseveral interacting factors. The presence of a large number of smallsuspended or dispersed particles provides a large surface area closelyassociated with impurities still dissolved in the water, so that uponcooling the water in the heat exchange apparatus impurities tending toprecipitate will preferentially precipitate upon the surface areaprovided by the suspended or dispersed particles as opposed to thesurfaces of the heat exchange apparatus. This is especially true wherethe particles are of the same chemical constituency as the dissolvedimpurities, due to a seeding effect. Furthermore, where the dispersedparticles are of the same constituency as the dissolved impuritiestending to precipitate, the particles will also be of the same natureand charge as any material which has deposited in the heat exchangeapparatus. Thus, the dispersed particles will show a reduced tendency toassociate with deposits in the apparatus.

While the method and apparatus described in the copending applicationprovide a means for extracting heat from hot unrefined water whichavoids the significant problem of scaling in the heat exchangeapparatus, it is still desirable to improve the efficiency of thismethod and system, e.g. to decrease the amount of deposits upon theporous material used in this system. Furthermore, it is desirable toseek means for preventing undesirable deposits in the well casing andpiping employed for bringing water from an underground source to theheat exchange apparatus. Similarly, it would be desirable to limit thedanger of precipitation of impurities from hot unrefined water to theextent that the unrefined water could be passed directly intoconventional heat exchange apparatus, without the necessity ofsubjecting the water to conventional purification techniques which areexpensive and inefficient.

Summary of the Invention

It is, therefore, an object of the present invention to provide a systemof extracting heat from hot unrefined water by passing said hotunrefined water directly through conventional heat exchange apparatus.

Another object of the invention is to provide a system which permits thehot unrefined water to be passed directly through heat exchangeapparatus without subjecting the water in advance to expensive andinefficient purification techniques.

It is another object of the invention to provide a system for extractingheat from hot unrefined water wherein scaling and other solid build-upof impurities from the water is prevented in the hot water well casingand water transport equipment as well as in the heat exchange equipment.

A further object of the invention resides in the provision of a systemfor extracting heat from hot unrefined water wherein the agents forbeneficially treating said water are derived directly from the source ofhot unrefined water.

The invention also has a further object the provision of an improvedindirect contact system of extracting heat from unrefined water whereinthe cycle life of porous beds of material utilized as preliminaryindirect contacting means is substantially lengthened.

Thus, in accomplishing these and other objects, there has been providedin accordance with the present invention an improved method forextracting heat from hot unrefined water containing scale-formingdissolved and dispersed impurities wherein the hot unrefined water iscontacted with a heat exchange surface or surfaces. These heat exchangesurfaces may be the surfaces of a conventional heat exchanger such as atube and shell or they may be the surfaces of the porous material in anaccumulator-type heat interchanger. The improvement involves adding tothe hot unrefined water prior to its contact with the heat exchangesurface an agent capable of increasing the formation ofnon-scale-forming species of said scale-forming impurities wherebyscaling and other solid build-up on said heat exchange surface,particularly upon cooling of the water, is minimized. Non-scale-formingspecies are those which remain in solution or suspension in theunrefined water as it is passed through the heat exchange apparatuswithout forming scale and/or those which are harmlessly precipitated,e.g. solid non-scale particles which are small enough to remain insuspension in the moving water and be carried out of the heat exchangeapparatus thereby.

In one embodiment of the invention, there is added as the aforesaidagent, suspendible particles of finely divided solid material,preferably in the form of a suspension, which may be a slurry ordispersion, and more preferably, particles having the same or similarconstituency as at least some of the impurities in the unrefined water.In carrying out this preferred aspect of the invention, the particlesare collected from cooled unrefined water subsequent to its contact withthe heat exchange surface, and this is achieved preferably by directingthe cooled unrefined water into a detention receptacle where it is helduntil it is stabilized by natural cooling and settling of precipitatesand other solids of the impurities originally dissolved and dispersedtherein. Some additional generation of non-scale solids, e.g.precipitation of dissolved substances, may occur at this point due tofurther cooling of the water. At the bottom of the detention receptaclethere is formed a portion of the water which is rich in these solidparticles of impurities, and it is from this portion of the receptaclethat the aqueous suspension of particles is collected and added to thehot unrefined water.)

In another embodiment of the invention, the preliminary accumulator-typeheat interchange technique described in the aforementioned co-pendingapplication is employed which comprises passing a volume of hotunrefined water through a housing containing porous material whereby theheat of the unrefined water is given up to the porous material, passinga volume of clean liquid through the housing whereby the heat of theporous material is given up to the clean liquid and thereafter,preferably, passing the heated clean liquid through a heat exchangerwhereby the heat of the clean liquid is extracted. In connection withthis embodiment, there is used as the clean liquid at least in partwater from a substantially impurity-free portion of water which isformed at the surface of the previously defined detention receptaclewhich receives the unrefined water after it is passed, in this case,through the housing containing porous material.

In yet a further embodiment, there is provided a method wherein the hotunrefined water is derived from an underground geothermal cavity and theaddition of finely divided particles to the hot unrefined water isaccomplished by adding the particles to the water underground,preferably by injecting a suspension of particles directly into the boreof a geothermal well. Most preferably, the particles are added at apoint in the well below a point where the hot unrefined water isbecoming supersaturated by being cooled and/or concentrated as, forexample, by being partly converted into steam.

In another embodiment there is provided a method wherein the agent addedto the hot unrefined water comprises a reagent capable of generatingnon-scale solids of the impurities in situ. In the case of dissolvedimpurities this reagent may be capable of causing precipitation of apart of the dissolved impurities, preferably in the form of finelydivided particles. In the case of dispersed colloidal impurities, thereagent may be one capable of causing agglomeration of part of thesecolloidal impurities. In either case this reagent is preferably addeddirectly to the water while it is still underground in a geothermalwater well. A similar embodiment comprises adding to the hot unrefinedwater an agent capable of increasing the solubility of at least some ofthe dissolved impurities and/or of decreasing the degree of dispersionof some of the colloidal impurities, for example, an agent capable ofraising the pH of the water. Again, the addition of such agents may takeplace directly in the geothermal water well bore. Still another similarembodiment comprises adding a chelating agent to the water.

There is also provided in accordance with the present invention a systemfor extracting heat from hot unrefined water containing dissolved anddispersed impurities comprising a source of hot unrefined water, a heatexchange means having an entrance end and an exit end, a means forconveying water from the source to the entrance end of a heat exchangemeans, a detention receptacle for receiving cooled unrefined water fromthe exit end of the heat exchange means and a means for recycling asuspension, which may be a slurry or dispersion, of solid particles ofthe impurities from the cooled unrefined water in the detentionreceptacle for addition to the hot unrefined water at a point prior toits entry into the heat exchanger. The heat exchanger may compriseeither a conventional indirect contact heat exchanger, such as a shelland tube counterflow-type exchanger, or an accumulator-type heatinterchange system comprising at least one housing containing porousmaterial together with a source of clean liquid, means for selectivelyconnecting the source of unrefined water and the source of clean liquidthrough the entrance end of the housing, a heat exchanger for extractingheat from the clean liquid and means for selectively alternatelyconnecting the exit end of the housing to the detention receptacle andto the heat exchanger. Preferably, the source of clean liquid is atleast partially comprised of the substantially impurity-free water whichis produced in the detention receptacle. In the most preferred aspect ofthis embodiment, there is also provided a second accumulator-type heatinterchanger comprising a second housing containing porous material,which housing is of substantially the same size as the first housing andalso has an entrance end and an exit end, means for selectivelyconnecting the source of unrefined water and the source of clean waterto the entrance end of the second housing, and means for selectivelyalternately connecting the exit end of the second housing to thedetention receptacle and to the heat exchanger. Furthermore, it isadvantageous to provide for recycle of the suspension of particlesdirectly into a geothermal well supplying the hot unrefined water.

In another preferred aspect of the system, the detention receptaclecomprises at least two separate zones, including a first zone adaptedfor first receiving the cooled unrefined water and for permitting largerparticles of impurities to settle, and interconnected therewith, asecond zone adapted for receiving the cooled unrefined water subsequentto the first zone and for permitting finer particles of impurities tosettle. The recycle system thus communicates with the second zone of thedetention receptacle and withdraws a suspension or slurry comprising thefiner particles.

Other objects, features and advantages of the present invention willbecome apparent from the detailed description which follows, whenconsidered together with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of the system of the invention,illustrating particularly the concept of a multi-zone detentionreceptacle.

FIG. 2 is a schematic flow diagram of another embodiment of the systemof the invention, illustrating provision for injection directly into ageothermal well.

FIG. 3 is a schematic flow diagram of another embodiment of theinvention wherein a preliminary, heat accumulator exchange technique isemployed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Whereas it was discovered in connection with the subject matterdisclosed in co-pending application Ser. No. 424,470 discussedhereinabove that the presence of minute suspended solid particles in theclean liquid being circulated to the heat interchanger providessignificant advantages, namely, a reduction of scale in the heatexchanger, it has now been discovered in accordance with the presentinvention that the presence of such solid particles in the unrefined hotwater stream likewise gives rise to significant benefits in a systemwhere heat is being extracted from the unrefined hot water, either byconventional heat exchange or heat accumulator interchange techniques.In fact, the presence of these dispersed particles renders it practicalto direct the stream of hot unrefined water directly into a heatexchanger without subjecting the water stream to any purificationprocedures. It has also been discovered in accordance with the inventionthat certain other agents may be added to the hot unrefined water streamto yield the same effect of reducing the amount of scale formation dueto impurities in the water. For example, it has been found that chemicalagents may be added to the hot water stream which give rise to theproduction of non-scale-forming species, often in the form of finelydivided particles, in situ. Other chemical agents may be added to thehot water which decrease the degree of saturation of the dissolvedimpurities or decrease the degree of dispersion of colloidal impuritiesby other mechanisms. For example, an agent which causes an increase ordecrease in the pH of the water may be employed. Chelating agents canalso be used. The addition of any of these agents may take place at anypoint prior to introduction of the unrefined hot water into heatexchange apparatus; however, as shall be discussed hereinbelow, it hasbeen found particularly advantageous to introduce these agents directlydownhole into the well bore of a geothermal water well.

As discussed briefly hereinabove, the beneficial effect resulting fromthe presence of solid particles suspended in the unrefined hot waterwith regard to the dissolved impurities is believed to be the result ofthe large amount of intimately located surface area provided by theseparticles for the reception of the dissolved impurities as theyprecipitate from the water as a result of changes in temperature and/ortheir concentration in the water. The suspended solid particles may alsocollect dissolved impurities by acting as seed crystals, particularly ifthey are of the same composition as the dissolved impurities or have asimilar crystalline structure. The suspended solids are also beneficialwith regard to other impurities present in the water, for example, inthe form of colloidal dispersions. The suspended solids collide with thedispersed colloidal impurities in the water and collect these impuritieson their surfaces thereby reducing the concentration of the colloidalimpurities. The suspended solid particles may even actively attract thecolloidal impurities, depending on their composition. For example,particles which have a charge opposite that of the colloidal impuritiesmight be chosen.

Thus, the water from geothermal sources is typically saturated and mostoften becomes supersaturated with dissolved impurities such as silica,calcium sulfate, silicates and other salts as it emerges from the hotwater well. The water also typically contains impurities such as silica,silicates, and iron in the form of colloidal dispersions and inconcentrations sufficient to cause scaling and solid build-up problems.As the water is cooled down, particularly in the heat exchangeapparatus, there is a great tendency for dissolved impurities toprecipitate from solution and the dispersed colloidal impurities to dropout, and normally there is encountered an intolerable build-up ofdeposits upon the heat exchange surfaces.

Furthermore, the hot mineral water from a geothermal well is preferablyat a temperature on the order of 400° to 500° F., at which temperatureit has a vapor pressure on the order of 200 psi. Deep down in the wellbore the hydrostatic pressure is high; however, it is significant tonote that as the water emerges from the well and rises near the surfaceof the ground, the hydrostatic pressure, or the pressure applied by thebottom hole pressure, becomes increasingly lower and the hot mineralwater may boil or flash. In some cases it has been attempted to avoidthis boiling by pressurizing the water with a pump submerged a fewhundred feet into the well, but since a submerged pump is inconvenientand expensive to operate and maintain, the hot mineral water is simplyallowed to boil in the well bore near the surface in order to reduce thedensity of the upper portion of the water column in most methods ofproducing hot mineral water. The water will then flow with theliberation of steam, which results not only in a cooling of the watersupply but also in concentration of the water to the extend of the waterremoved by formation of steam. Such cooling and concentrationcontributes to the supersaturation of dissolved impurities in themineral water and further concentration of the dispersed colloidalimpurities, and furthermore, a significant problem arises as a result ofthe tendency of the impurities to form scale in the upper portion of thewell casing as well as within the piping utilized to convey the waterfrom the well to heat exchange equipment. In accordance with one aspectof the present invention, this supersaturation and tendency for scaleforming is eliminated or at least significantly reduced by adding one ormore of the agents according to the invention to the hot mineral waterat a point down in the well which is below a point at which the water isboiling or flashing. The addition may be made simply by introducing theagent down in the well bore by means of a tube extending the requireddistance down into the well. This treatment can be used alone or toaugment similar treatment of the unrefined hot water immediatelyupstream of the ultimte heat exchange equipment utilized to extract heattherefrom.

It should be noted that in most practical cases at least some scale willresult on the surfaces of the heat exchanger and/or the well casing andtransport piping. Since in a preferred embodiment of the invention, theparticulate material added to the hot water stream is of the sameconstituency as the scale which has deposited from the stream, i.e.since the particulate material is preferably collected from the mineralwater stream after it has been cooled or is formed in situ directlywithin the hot water stream, the suspended solid particles will have thesame electrical charge as the scale, and, therefore, the particles withthe impurities they collect on their surfaces will not have a tendencyto be attracted to the scale and they thereby resist the formation offurther scale. It is to be understood, of course, that any type ofparticles added to the hot water stream may be caused to have the samecharge as the scale which tends to deposit from the stream, for example,by treatment with an appropriate agent. Furthermore, the hot waterstream containing solid particles suspended therein tends to actsomewhat in an abrasive manner, thereby offering yet anotheradvantageous means for preventing or minimizing the deposition of scalein the apparatus.

As indicated, the preferred method of inroducing finely dividedparticles into the unrefined hot water streams involves adding asuspension, which may be a slurry or dispersion, of sediment collectedfrom cooled mineral water from the same or a different undergroundsource. Other types of particulate material suitable within the contextof the invention include ground or precipitated limestone or clays.

In addition to suspensions of finely divided solid particles, otheragents may be added to the hot unrefined water to increase the formationof non-scale-forming species of the scale-forming impurities naturallypresent in the water. These include reagents capable of generatingnon-scale solids in situ. Some reagents will cause the precipitation ofdissolved impurities by the common ion effect. A reagent is added, oneion of which will combine with one or more of the ions of the dissolvedimpurity to form a compound which is less soluble than the dissolvedimpurity. This new compound will then precipitate harmlessly in the formof small solid particles which do not attach to the surfaces of theequipment and which can be carried along by the flowing water, and thedegree of saturation or supersaturation of the water will be decreased.For example, the addition of sodium carbonate causes precipitationaccording to the following equation:

    CaS0.sub.4 (dissolved) + Na.sub.2 C0.sub.3 → CaC0.sub.3 + Na.sub.2 S0.sub.4

and the addition of sulfate ion by use of a soluble sulfate compount,e.g. Na₂ S0₄, produces the following reaction:

    CaS0.sub.4 (dissolved) + S0.sub.4 - → CaS0.sub.4 + So.sub.4 -

as a result of the effect of the added sulfate ion (S0₄ -). Otherreagents giving rise to precipitations of dissolved impurities will bereadily apparent to those skilled in the art.

Reagents may also be used to generate non-scale solids of impurities incolloidal dispersion by causing agglomeration of these impurities intonon-scale solid particles small enough to be carried by the water butlarge enough to be readily separated from the water, as by settling inthe detention receptacle which receives the unrefined water after it haspassed through the heat exchange apparatus.

Scaling problems from dissolved impurities can also be prevented byincreasing the solubility of the dissolved solute, thereby alsodecreasing the degree of saturation with respect to this solute.Similarly, scaling problems of colloidal dispersions can be prevented bydecreasing the degree of dispersion of the impurities. For example, thesolubility of Si0₂ in the hot unrefined water may be increased by theaddition of an alkaline compound, for example, caustic, which increasesthe pH of the solution. The degree of dispersion of a colloidal impuritycan also be decreased by the addition of an agent which changes the pHof the unrefined water. Other examples of agents which can be used toalter the pH are NaOH and Ca(OH)₂.

Another type of agent which may be used to increase the formation ofnon-scale-forming species of the impurities, both dissolved anddispersed, is a chelating agent such as one of the chelating agents foriron.

Referring now to FIG. 1, hot underground water is drawn up from a well101. The hot unrefined water is delivered by a line 103 into theentrance end of a heat exchange apparatus 102, for example, into thetube side of a conventional heat exchanger. After passing through theheat exchanger, the unrefined water, now in a cooled state, is conveyedthrough line 104 into an aging and settling pond referred to generallywith the reference numeral 105. The hot water is cooled in heatexchanger 102 by indirect contact with another fluid on the shell sideof the heat exchanger whereby heat utilizable in a conventional heatengine or other power cycle is produced. The hot water, after beingcooled in heat exchanger 102 is permitted to cool and stabilize furtherin settling pond 105, after which it is discharged from the settlingpond by line 106 and is returned to the ground into water injection well107.

The cooled water exiting heat exchanger 102 contains suspended solidparticles which, in part, result from precipitation of the dissolvedimpurities and agglomeration of the dispersed colloidal impuriites inthe hot water as a consequence of the temperature change which the waterundergoes. The suspended solid particles settle as the water ispermitted to remain in the settling pond 105 and the particlesaccumulate at the bottom of the pond. A slurry of these particles iscollected at the bottom of the pond and is pumped by pump 108 positionedin recycle line 109 from the settling pond, and this slurry of finelydivided particles is then injected into the hot unrefined water streamat pont 110 which lies upstream of the inlet to the heat exchanger 102.Dispersing of the slurry of particles into the hot water stream providesthe suspended particles, the benefits of which have been describedhereinabove.

An agent or reagent may be added to the cool unrefined water after ithas passed through heat exchanger 102 but prior to the collection ofthis slurry to increase the generation of the slurry particles,particularly in cases in which the particles already present in thewater are insufficient in quantity or otherwise unsuitable for use as anagent to be injected into the hot water in accord with the presentinvention. Such agents include reagents which will cause thepecipitation of solutes in the water by the common ion effect asdescribed hereinabove. For example, the hot unrefined water may containsolutions which are not sufficiently saturated to present scalingproblems as they pass through the heat exchange apparatus. Thus, therewould be no need to inject an agent operative upon solutes of theseparticular dilute solutions into the hot water prior to its entry intothe heat exchange apparatus. These solutes may, however, be desirable ascomponents of the slurry particles to be collected from the detentionreceptacle. Thus, a reagent operative upon the solutes in question mightbe added to the receptacle prior to the collection of the slurry.

Continuous recycling of these fine particles will inevitably lead to theresult that the particles become larger and larger due to thescale-forming material which is deposited thereon during the coolingstep. However, as a result of the erosion caused by abrasion between theparticles themselves and the apparatus with which they come intocontact, and also because of the formation of new precipitation oragglomeration nuclei, a certain amount of sufficiently finely dividedparticles will always remain in the system. Furthermore, in thedetention pond 105 the larger particles tend to settle first, whereasthe finer particles require a longer period of time to settle. Thus, bya proper choice of recovery zone within the detention pond, it ispossible to selectively recirculate back to the incoming hot waterstream only the finer solid particles. In this manner, the process maybe operated continuously. The term "finely divided particles" isintended to imply particles of a size which may be suspended in theincoming hot water stream.

Referring also to FIG. 1, the settling or detention pond 105 depictedtherein illustrates a further feature of the invention according towhich the detention receptacle is provided with two distinct,interconnected zones designated by the reference numerals 112 and 113.The first zone 112 contains the inlet to the detention pond and receivesthe cooled water from heat exchanger 102 through line 104. This firstzone provides a settling area for the larger size particles which haveaccumulated in the system, in order that these particles may beeliminated from the slurry recirculation system. The water being held inthe detention pond then passes from the first zone 112 into the secondzone 113, wherein the finer particles begin to settle as the detentiontime for the water is increased. Slurry pump 108 and recycle loop 109communicate with the second zone 113 in order that the fine particlescollected therein may be recirculated to the incoming hot water streamin line 103.

In FIG. 2 of the drawings, there is illustrated a system embodying theessential elements shown in FIG. 1 and the identical elements have beenidentified with equivalent reference numerals. In structure andoperation the system illustrated in FIG. 2 is identical to that of FIG.1 except insofar as the recycle arrangement is concerned. In the systemof FIG. 2, the slurry of finely divided precipitated particles isremoved from the detention pond 105 via line 109 containing a slurrypump 108. But in this instance, line 109 terminates in a length of pipe114 which extends a substantial distance down into the bore of hot waterwell 101. In this way, the slurry of fine particles is injected directlyinto the hot water well. Alternatively, any of the other agentsdiscussed hereinabove which are effective for increasing the formationof non-scale-forming species of the impurities in the hot water may beintroduced into the recycle system via line 115, either exclusively orin conjunction with the slurry being provided from detention pond 105.

Referring now to FIG. 3, hot underground water is drawn up from a well10. The hot unrefined water is delivered via line 12 to a four-way valve14. When valve 14 is in the position shown in solid lines, the hot wateris delivered to the entrance end 22 of a container 16 containing aporous, preferably gravel bed 18. The gravel bed 18 absorbs the heat ofthe unrefined water as it flows through the housing 16. Meanwhile, avolume of cool clean water or other suitable liquid is pumped from aheat exchanger 38 by pump 46 into the entrance end 26 of a secondhousing 30 via line 48 and four-way valve 14. The housings 16 and 30 arepreferably of the same size and housing 30 contains gravel bed 32. It isto be understood that the gravel 32 in the first half of the housing 30at the beginning of this step in the cycle is hot, a volume of hotunrefined water having previously been passed therethrough. The secondhalf of the gravel 32 in housing 30 is cool at this point as theunrefined water contained in its voids has given up its heat to thefirst half of the gravel. Thus, a temperature front is moving throughthe gravel with half the velocity of the water itself and, at thisstage, is located at the middle of the gravel 32. Then, as a volume ofclean water begins to flow through the housing 30, it picks up the heatof the first half of the gravel 32 simultaneously driving the hotunrefined water out of the first half of gravel. The latter hotunrefined water now heats the second half of the gravel 32 while itselfis cooled.

The volumes of unrefined and clean water are admitted to the respectiveentrance ends 22 and 26 of the housings 16 and 30 at approximately thesame time. These volumes of water have substantially the same flowcharacteristics so that they flow through their gravel beds atsubstantially the same speed. Each of these volumes of water may beconsidered to have a leading edge, i.e. the portion of the volume ofwater which has passed farthest toward the exit end 24 or 28 of itsrespective housing. The leading edges of the two volumes reach the exitends 24 and 28 at approximately the same time, and the portions of therespective volumes of water which are then at the entrance ends 22 and26 may be considered the trailing edges of their respective volumes ofwater. The leading edge of the clean water reaches the outlet of thehousing 30 at the same time as the above-mentioned high temperaturefront, and a second low temperature front is now located mid-way throughthe housing.

At this time valve 14 is switched to the position shown in dotted linesso that a new volume of unrefined water begins to flow into bed 32, thefirst half of which is now cool, and a new volume of clean water beginsto flow into bed 18, the first half of which is now hot. Simultaneously,a second four-way valve 20 connected to the exit ends 24 and 28 of thebeds is turned to the position shown in dotted lines so that the coolunrefined water now exiting from housing 16 is delivered to a detentionreceptacle such as a pond 34 and the clean water which has been heatedin bed 32 is delivered to a heat exchanger 38 or to a boiler or the likewhich may be more or less conventional. Here it gives up the heatabsorbed from the unrefined water by means of the gravel bed 32 to aworking fluid which ultimately results in the production of useful work.

As described more fully in the co-pending application Ser. No. 424,470,the gravel or other porous material in the housings 16 and 30 is chosenso that the tempeature fronts will move therealong with half thevelocity of the flowing water. In particular, the porous material ischosen so that its heat capacity per unit volume when dry issubstantially the same as the heat capacity of the unrefined water inthe voids of a unit volume of the porous material and also substantiallythe same as the heat capacity of the clean water in the voids of a unitvolume of the porous material.

It will be appreciated that each of the beds 18 and 32 has volumes ofunrefined water and clean water alternately passed therethrough in acontinuous cycle, the unrefined water giving up heat to the gravel andthe gravel giving up het to the clean water. It will also be appreciatedthat while unrefined water is passing into one bed, clean water ispassing into the other so that the total process of heat transfer inexchanger 38 is continuous. By proper switching of valve 20 theunrefined water exiting from one bed is directed into the detention pond34 while the clean water is entering that bed and clean water exitingfrom the other bed is directed to the heat exchanger 38 while unrefinedwater is entering said other bed. The solid line position of valve 20directs water from housing 16 to the heat exchanger 38 and water fromhousing 30 to the pond 34; the dotten line position directs water fromhousing 16 to the pond and water from housing 30 to the heat exchanger.

In the preferred form of the invention, the unrefined water which entersthe pond 34 from the housings 16 and 30 has been cooled by giving up itsheat to the gravel in the beds. It is then in condition to attainstabilization by further elimination of its supersaturation and settlingof the precipitated and agglomerated particles. As it rests in the pond34, many of the impurities settle to the bottom of the pond leaving onthe top of the pond a liquid which is substantially free of impurities.By "substantially free of impurities" is meant that the liquid issufficiently free of impurities that it can be safely passed through aheat exchanger or other apparatus in direct contact with the surfaces ofthe apparatus without excessive danger of scaling, corrosion, etc. Forexample, it should be less than saturated with any dissolvedscale-forming impurities so that it will not become supersaturated withsame when it is heated in the heat exchange apparatus. If necessary, thewater in the detention receptacle may be treated, as with a suitableagent or reagent, to achieve this "substantially impurity free" state.However, the cooling of the water in the detention receptacle, togetherwith whatever agents were added prior to its entry into the heatexchange apparatus, is usually sufficient to accomplish this end. It ispart of this substantially impurity-free liquid that is used as at leastpart of the "clean water" while the remainder of the impurity freeliquid may be returned to the ground via line 50 and well 36. It will beunderstood that part of the clean water may come from another source,however, the substantially impurity-free liquid in the pond may, andpreferably does, supply all the clean water.

The fine particles of impurities precipitating to the bottom of thedetention pond are removed via line 9 containing therein a slurry pump8, the particles being removed in the form of a dispersion or slurrythereof in water. This dispersion or slurry is returned via the recycleline 9 to line 12 which transports the hot unrefined water from the hotwater well 10 to the porous bed-packed housings 16 and 30. Thus, whileit is true that the gravel in beds 18 and 32 is usually inexpensive andexpendable, these beds may become fouled with impurities as a result ofoperation of the system and, therefore, must be periodically replaced orreconditioned for reuse by tumbling and washing or the like. However, asa result of the feature of recycling a slurry or dispersion of finelydivided solid particles in the recycle line 9, the frequency with whichthe gravel in beds 18 and 32 need be replaced or reconditioned isconsiderably lessened, and in some cases this requirement is completelyavoided. It is to be understood in connection with this embodiment ofthe invention that the slurry or dispersion of particles recycled vialine 9 may be also injected directly into the hot water well 10, andfurthermore, that other agents defined herein as being suitable forincreasing the formation of non-scale-forming species of the impuritiesdissolved in the hot water may likewise be injected into the hot watersupply, either in hot water well 10 or in supply line 12.

It is also to be understood that, since there is some contact betweenthe unrefined water and clean water in the housings and also becausesome impurities are left in the housings by the unrefined water andpicked up by the clean water, the clean water eventually begins toaccumulate impurities. As described more fully in the co-pendingapplication, the clean water also contains a small amount of dispersedparticles similar to those which are collected from the pond 34 andinjected into the unrefined water via line 9. These particles tend tocollect on their surfaces many of the impurities which are introducedinto the clean water.

Both the dispersed particles from the pond and the impurities which areaccumulated in various forms in the clean water are usually present insmall amounts and in the form of non-scale-forming species. Thus, theirpresence in the clean water is essentially harmless. However, as thequantity of these substances increases, it may be desirable to freshenthe clean water. The clean water may be freshened by continuously orperiodically tapping off quantities of the clean water fromcommunication with the heat exchanger 38 and other parts of the systemvia line 44 and returning it to the pone 34 for restabilization. Thesequantities of the clean water are replaced by quantities ofsubstantially impurity-free liquid from the pond 34. These quantities ofliquid may be pumped from return line 50 by pump 40 and directed intocommunication with the heat exchanger and other parts of the system vialine 42.

Since it is apparent that many modifications of the system and method ofthe invention will suggest themselves to those skilled in the art, it isintended that the scope of the invention be defined solely by the claimsappended hereto.

I claim:
 1. A system for extracting heat from hot unrefined watercontaining scale-forming impurities comprising:a source of said hotunrefined water; a heat exchange means having an entrance end and anexit end; means for conveying said hot unrefined water from said sourceto the entrance end of said heat exchange means; a detention receptaclefor receiving cooled unrefined water from the exit end of said heatexchange means; and means for separating and extracting a dispersion ofprecipitated particles of said impurities from said cooled unrefinedwater in said detention receptacle, and recycling said dispersion tosaid hot unrefined water at a point prior to its entry into said heatexchange means.
 2. The system as defined by claim 1 wherein said heatexchange means comprises at least one housing containing porousmaterial, and said system further comprises a source of clean liquid,means for selectively connecting said source of clean liquid, means forselectively connecting said source of unrefined water and said source ofclean liquid to the entrance end of said housing to alternately pass avolume of said unrefined water and a volume of said clean liquidtherethrough; a heat exchanger for extracting heat from said cleanliquid; and means for selectively alternately connecting the exit end ofsaid housing to said detention receptacle and to said heat exchanger topass unrefined water exiting from said housing into said detentionreceptacle and to pass clean liquid exiting from said housing throughsaid heat exchanger.
 3. The system as defined by claim 2 wherein aportion of the impurities in said unrefined water settles in saiddetention receptacle leaving a substantially impurity-free liquid at thetop of said receptacle and wherein said source of clean liquid is atleast partially comprised of said substantially impurity-free liquid,said clean liquid being clean water.
 4. The system as defined by claim 2wherein said clean liquid is clean water and said system furthercomprises a second housing containing porous material, said secondhousing being substantially the same size as said first housing andhaving an entrance end and an exit end; means for selectively connectingsaid source of unrefined water and said source of clean water to theentrance end of said second housing to alternately pass a volume of saidunrefined water and a volume of said clean water through said secondhousing; and means for selectively alternately connecting the exit endof said second housing to said detention receptacle and to said heatexchanger to pass unrefined water exiting from said second housing intosaid detention receptacle and to pass clean water exiting from saidsecond housing through said heat exchanger.
 5. The system as defined byclaim 1 wherein said source of hot unrefined water is an undergroundgeothermal cavity and said recycle means comprises means to inject saiddispersion into said water underground.
 6. The system as defined byclaim 1 wherein said detention receptacle comprises a first zone adaptedfor first receiving said cooled unrefined water and for permittinglarger particles of said impurities to settle, and interconnectedtherewith, a second zone adapted for receiving said cooled unrefinedwater subsequent to said first zone and for permitting finer particlesof said impurities to settle.
 7. The system as defined by claim 6wherein said means for recycling a dispersion of precipitated particlesis in communication with the second zone of said detention receptacle.8. The system as defined by claim 3 wherein the heat capacity per unitvolume of said porous material when dry, the heat capacity of theunrefined water in the voids of a unit volume of said porous material,and the heat capacity of the clean water in the voids of a unit volumeof said porous material are substantially equal.